FR2503377A1 - DEVICE FOR MEASURING VARIATIONS IN THE GAIN OF A LASER AMPLIFIER BASED ON PUMPING ENERGY AND METHOD USED THEREFOR - Google Patents

Abstract

THE OBJECT OF THE INVENTION IS A PROCESS AND A DEVICE FOR MEASURING THE GAIN OF A LASER AMPLIFIER AS A FUNCTION OF THE PUMPING ENERGY. ACCORDING TO THE INVENTION, WE DETERMINE THE GAIN GT AS A FUNCTION OF TIME AND SIMULTANEOUSLY THE ENERGY USED AND DURING PUMPING THEN WE ELIMINATE THE TIME BETWEEN THESE TWO FUNCTIONS, WHICH GIVES, IN A SINGLE DRAW, THE VARIATION GE OF THE GAIN IN FUNCTION OF PUMPING ENERGY FOR A GIVEN RADIUS OF THE AMPLIFIER BAR. APPLICATION TO LASER AMPLIFIERS.

Description

The present invention relates to a device for measuring variations

  of the gain of a laser amplifier as a function of the energy of

  pumping and a method implementing this

  tif. It finds an application in optics, especially

  in the production of amplification chains.

lasers.

  There are essentially two methods of measuring the gain of a laser amplifier. The

  the means of implementing them are

  shining schematically in Figures 1 and 2.

  In the device represented on the

  In FIG. 1, the laser amplifier whose gain is to be measured bears the reference 10. It consists of

  classically by an amplifier bar 12

  tower of which is wound a flash tube 14 fed

  in electrical energy by a circuit 16.

  The device further comprises a laser

  oscillator 20 capable of emitting light radiation

  impulse-shaped 22 which crosses the bar-

  amplifier 12 at a distance R n from the axis of this bar, this radiation being at a wavelength which is the one at which the measurement of the gain must be made. The pulse 22 is amplified

  by the bar 12 when it is made ampli-

  under the effect of the optical pumping created by

  the flash tube. The output pulse 24 which appears

  at the output of the amplifier then has a higher energy than the impulse

22.

  The system still includes two blades

  half-transparent 31 and 32 placed respectively at the input and at the output of the amplifier bar 12

  and two photosensitive detectors 33 and 34 (heat

  meters, photocells, etc ...) suitable for

measure a light energy.

25033 ??

  The operation of such a device is as follows. The blades 31 and 32 take part of the input pulses 22 and output 24 and direct this portion on the detectors 33 and 34 which measure the corresponding energies. The

  measurement of the ratio of these energies gives immediate

  the gain of the bar 12, for the value of the pumping energy used and for the radius Rn considered. To obtain the G (E) variations of the gain

  depending on the pumping energy E, it is necessary to

  carry out a series of such measures for several

sieurs values of E.

  The second method is illustrated on the

  FIG. 2 shows that the device shown also comprises an amplifier laser 10 whose measurement is to be measured

  gain, and a laser oscillator 40 which, unlike

  laser 20 of the previous device, function

  continuously. This laser emits radiation to

  the amplification wavelength of the bar 12.

  When the latter is for example a glass rod doped with neodymium, the laser 40 may be

example of the YAG type.

  The device shown still comprises

  a separating blade 42, a photoelectric detector

  that 44 and a divider 46 of the signal issued by 44

  by a quantity proportional to the power de-

delivered by the laser 40.

  The operation of this device is illustrated by means of the graph of FIG.

  represents the variation over time of the signal de-

  This signal represents the light power Ps (t) measured at the output of the amplifier 10 for the radius Rn. In the absence of optical pumping, this power is equal to the

  the Pe value of the input radiation emitted by the

ser 40.

  While pumping, the power Ps (t)

  increases to a maximum value Pm, which is

  tint at a moment tm, then decrease to tend again to Pe. The gain of the amplifier bar, which is equal to the quotient Ps (t) / Pe, therefore varies in

  function of time throughout the duration of the

  pumping pulse and goes through a maximum equal to

Pm / Pe and reached at time t = tm.

  To obtain the variations of the gain as a function of the pump energy E, it is necessary, as for the previous device, to carry out several measurements for different energies E and to record each time the value of the corresponding gain, by

example the maximum gain.

  This method presents on the previous

  the advantage of allowing the determination of the

  as long as the gain is maximum, which makes it possible to

  chronicle the different amplifiers of a

laser chain.

  The two processes of the prior art which have just been described have the disadvantage of requiring several pumping operations or

  shots "to explore the entire range of energy

  haitée. Obviously, this constraint increases the measurement time. In addition, it reduces the duration

of the element to be measured.

  The subject of the present invention is precisely a device and a method which avoid these two drawbacks because it makes it possible to obtain the desired variations of the gain as a function of the pumping energy in a single shot, that is to say in one only pumping operation. For this purpose, the device is of the kind described in connection with FIG. 2 and is characterized in that it further comprises means for delivering a signal E (t) representative of the croissanze of the pumping energy used during pumping and a means

  to draw two signals G (t) and E (t) by eliminating

  nation of the time t, a signal G (E) which represents the variations of the gain as a function of the pumping energy. The present invention also

  object a process for the implementation of the

  tif which has just been defined. This method consists first of all (and as in the second method of the prior art described above), to record the variations G (t) of the gain as a function of time for a fixed pumping energy and, unlike

  the prior art, to simultaneously identify the variations

  pumping energy E (t) used throughout the pumping pulse, then to be determined

  the variations G (E) of the gain as a function of the energy

  by eliminating the time parameter between

  two functions G (t) and E (t). We thus obtain

  and in a single shot the variations of the gain in the interval between zero and the total energy used. The latter must naturally

  be chosen large enough for the inter-

  The valley in question covers the desired beach.

  Different particular modes of

  of the invention will now be described with reference to the accompanying drawings which follow

  those of Figures 1 to 3 already described and on-

  which: FIG. 4 is a diagram illustrating the variations of the gain of a laser amplifier in

  a function of time, for a given pumping energy

  FIG. 5 is a diagram illustrating the variations of the excitation voltage of the flash tube as a function of time; FIG. 6 is a diagram illustrating the variations of the excitation current of the flash tube as a function of the FIG. 7 is a diagram illustrating the variations of the electrical power dissipated in the flash tube as a function of time, FIG. 8 is a diagram illustrating the variations of different energies involved, and in particular the pumping energy, FIG. 9 represents the variations of the gain as a function of the pumping energy for a radius Rno

  - Figure 10 represents schematic

  1a and 11b show a variant with a plurality of photoreceptors distributed along a radius of the amplifier, FIG. 12 represents means for carrying out the process of the invention, FIGS.

  additional treatment to determine

  the variations of the gain as a function of the radius.

  As mentioned above, the determination

  according to the invention variations of the gain in

  pumping energy function passes through the de-

  preliminary determination of the variations G (t) of the gain as a function of time and those E (t) of the energy

  pumping according to this same time. The dia-

  gram of Figure 4 represents the first, ie G (t), in a system of axes where the time is in abscissae (scale 0.1 ms per division) and the gain G in ordinates (and expressed in dB) and

  this for a particular amplifier. This cour-

  be is obtained from the results illustrated by

  Figure 3, taking the value of the gain as the

  holds the output power Ps (t) of the amplifier

  by the input power Pe. This first 25033t7

  determination can be made as in the ancient art of

  TER AL. According to the invention, the variations of the voltage V (t) and the current I (t) applied to the flash tube are measured simultaneously. These variations are illustrated respectively in Figures 5 and 6 where time is still

  abscissa with the same scale (0.1 ms per

  voltage and current being carried

  ordered, always for the particular amplifier

  to link in question. The scale of the voltages is 1 kV by division and that of the currents of 5 kA by division.

  Figure 7 shows the electrical power

  P (t) obtained by formation of the product V (t) .I (t). The scale of the powers is, in the illustrated case, of 30 MW by division. That of time

  is always 0.1 ms per division.

  The integration over time of this power makes it possible to determine the growth of the energy, ie EF (t), dissipated in the flash tube. To obtain the total energy E (t), the energy EJ (t) dissipated by the Joule effect in the supply circuit and the tube must be added to EF (t).

  lightning and which does not participate in optical pumping.

  The variations of these three energies are

  shown in the diagram of Figure 8 o the time

  is always plotted on the abscissa (0.1 ms per

  sion) and energy on the ordinate (1 kJ per

if we).

  According to the invention, from the signals

  G (t) (Figure 4) and E (t) (Figure 8) are drawn by eli-

  the time t, a signal G (E), which is

  represented by the curve in Figure 9 where the energy

  E is plotted on the abscissa (1 kJ per division) and G gain on the y-axis (1 dB per division) for

  a radius Rn of the bar 12. In accordance with the invention

  This curve is obtained in a single shot.

  The means making it possible to implement the method which has just been described are illustrated in FIG. 10. They comprise first of all and as in the prior art illustrated in FIG.

  -2, a source 40 of continuous radiation which functions

  at the measuring wavelength and which emits

  a power Pe. This radiation crosses the amplifier

  indicator 12 whose gain is to be measured. A way

  42 is used to collect some of the radiation

  amplified at the output of the amplifier, this

  medium being generally a semi-transparent blade.

  A photoreceptor 44 receives the sampled portion and measures the output power Ps throughout the

  duration of optical pumping. Finally, a circuit 46

  me the quotient of Ps by Pe and delivers a signal

  G (t) which represents the variations of the gain in function

time during pumping.

  The device of Figure 10 is

  characterized in that it further comprises means 50 for measuring the growth E (t) of the pump energy and means 52 for pulling the two signals G (t) and E (t), by eliminating the time t , the signal

G (E) sought.

  In the example illustrated, the means 50 com-

take:

  a circuit 54 for measuring the voltage V (t) of

  of the flash tube 14, a circuit 56 for measuring the current I (t) flowing in the flash tube,

  a multiplier 58 connected to the two preceding circuits

  teeth and delivering a product signal P (t) = V (t) .I (t),

  an integrator 60 connected to the multiplier 58 and

  delivering a signal EF (t) representing the pumping energy of the flash tube, a circuit 62 determining the energy EJ (t) lost by the Joule effect in the pumping circuit, by

  the determination of the product Ri2dt of the resistance

  this equivalent R, of the current i, during an inter-

  time interval dt and integration of this product, an adder 64 connected to the latter circuit 62 and to the integrator 60 and delivering the signal E (t) sought. The entire calculation chain described can

  be of an analogue character or

  or hybrid type digital-analog

than.

  In the above, the measurement light beam has a low power. Gain

  measured is a gain "small signal". In addition

  the diameter of this beam is generally much smaller than that of the laser amplifier, so that the measured gain is the gain at

  a definite distance from the axis (R) of the ampUifi-

  er. But it is easy to extend the invention to the case where the beam emitted by the oscillator laser occupies a large fraction of the cross section.

  of the amplifier bar, in which case the measure de-

  may be carried out simultaneously in more than one

points.

  It is particularly interesting to use a measuring beam whose diameter is equal to (or slightly greater than) the radius of the bar to be measured and receiving means distributed along a radius of the amplifier bar. This is shown in Figures lla and llb where we see

  a plurality of photoreceptors 70, 71, 72 ...

  departing along a radius at distances from the axis equal to R0, R1, R2, ... Rn These photodetectors may be photodiodes or elements of a

bar of the kind Réticon.

  FIG. 12 shows an ancillary processing means making it possible to make the most of the device of FIG. 11 with several photodetectors. The means 52 for eliminating time, receives, in addition to the signal E (t), as many signals G (t) as there are photoreceptors, or

  GRO (t), GRl (t), GR2 (t), etc ... Instead of only having

  der an output delivering a signal G (E), the

  medium 52 has as many as photoreceptors.

  These outputs delivering signals representing the variations GRO (E), GRi (E), GR2 (E), etc ... of the gain as a function of the energy for the different radii R0, R1, R2, etc. These The results are stored in means 100, 101, 102, etc. (see the curves on the right of the means 100, 101,

102, etc ...).

  Means 110, 111, 112, etc. connected directly to the means 100, 101, 102, etc. are capable of determining, for each radius R0, R1, R2,

  etc ... the value of the gain for a particular energy

  (E1). These different values are taken into account by a means 120 which thus determines, for

  said value E1 of the pumping energy, the values

  rations of the gain GE1 (R) as a function of the radius R (cf.

  the curve shown to the right of the means 120).

Claims (6)

  1. A device for measuring the gain variations of a laser amplifier (10) at a certain   wavelength, depending on a pump energy   page, this device comprising a source (40) de-   delivering continuous light radiation having   wavelength and a power Pe, this radiation   passing through the amplifier (12) to a de-   completed, means (42) for taking a portion of the   amplified radiation at the output of the ampli-   a photoreceptor (44) receiving this   and which measures the power Ps during the   duration of pumping, means (46) for forming the quo-   holds Ps by Pe and delivers a signal G (t) which represents the variations of the gain as a function of   during pumping, this device being   characterized in that it further comprises means (50) for providing a signal E (t) representing the growth of pump energy used during pumping and means (52) for pulling two signals G (t). ) and E (t), by elimination of the time t, a signal G (E) which represents the variations of the gain as a function of the pumping energy for the determined radius.   2. Device according to claim 1, characterized in that the means (50) for delivering the signal E (t) representing the pumping energy used during pumping comprises:   a circuit (54) for measuring the power supply voltage V (t),   a circuit (56) for measuring the feed current 1 (t),   a multiplier (58) connected to the two preceding circuits   teeth (54, 56) and delivering a product signal P (t) = V (t> .I (t),   an integrator (60) connected to the deli multiplier displaying an EF (t) signal,   a circuit (62) determining the energy EJ (t)   due to the Joule effect in the pumping circuit, - an adder (64) connected to the latter circuit (62) and to the integrator (60) and delivering the signal E (t) sought.   3. Device according to any one of   claims 1 and 2, characterized in that the   source (40) delivers a light beam whose   diameter is at least equal to the radius of the amplification   laser detector, the photoreceptor (44) comprising a plurality of photoreceptor elements distributed along the radius of the amplifier at distances R0, R1, R2, ... Rn from the axis of this amplifier,   these photoreceptors delivering (52)   GRO (t), GRi (t), GR2 (t), etc ..., this means de-   delivering so many signals GRO (E), GRi (E), GR2 (E), ... representing the variations of the gain for   the different values R0, R1, R2, etc ...   4. Device according to claim 3, characterized in that it further comprises means (100, 101, 102, etc.) capable of storing the signals GRO (E), GR1 (E), GR2 (E ), etc ... delivered by means (52), means (110, 111, 112, ...)   related to the above and able to determine the   their GRO (E1), GR1 (E1), GR2 (E1), etc ... taken by the gain for a particular energy E1, and a means (120) for determining the variations of the gain   GEl (R) along the radius of the amplifier for said energy E1.   5. Method for measuring variations of the   gain of a laser amplifier to a certain   as a function of pumping energy, a method employing the device according to the 12 2503377   claim 1 and wherein a continuous light radiation having said length is generated   wave and a power Pe, we cross the am-   by this radiation, we take a   At the output of the amplifier, the amplification of the amplified radiation is measured during the pumping period, and the quotient of Ps is calculated by Pe, which gives the variations G (t) of the gain as a function of time during the pumping, this method being characterized in that having fixed a total pump energy is also measured the growth E (t) of the pumping energy used during this pumping and in that one draws, by elimination of the time t between G (t)   and E (t), a relation G (E) which represents the values   sought-after gains in terms of energy pumping.   6. Process according to claim 5,   characterized in that, to measure the growth E (t) of the pumping energy used during pumping, the supply voltage V (t) is measured   and the supply current I (t), we form the   duit V (t) .I (t), this product is integrated into the   time, which provides an energy EF (t), we determine   mine the energy EJ (t) lost by Joule effect in the   pumping circuit and calculate the sum EF (t) + EJ (t).